The present invention is in the field of electrical power distribution and, more particularly, electrical energy storage systems that may provide temporary power to an electrical power distribution system of the vehicle or an industrial installation.
In many types of vehicles, electrical power is generated on board the vehicle and distributed to various electrical loads through a power distribution bus. In some vehicles, such as aircraft or other aerospace vehicles, power distribution systems are provided with a source of back-up electrical energy which may be used to maintain power on the bus in the event of loss of the main power sources or in the event of temporary overloading of the bus.
One type of back-up system employs so-called supercapacitors which may be charged and which may be connected to the bus as needed to function temporarily as a source of electrical energy to maintain a desired bus voltage.
In prior-art applications of supercapacitors, charging of the supercapacitors may require complex charging and conversion circuitry. This is because, in a discharged state, internal impedance of the supercapacitor may be as low as a fraction of a milliohm. Therefore, upon initial charging of the supercapacitor, a very large charging current may develop if charging is performed at the bus voltage. In order to limit charging current to acceptable levels, dedicated current-limiting chargers or other current-limiting devices may be employed. Dedicated current-limiting chargers or other current-limiting devices typically must absorb and dissipate a sizeable amount of heat. In this regard, the current-limiting devices must be relatively large. Their use on a vehicle such as an aerospace vehicle may add undesirable weight and cost to the vehicle.
Conversely, when the supercapacitor is discharged onto a bus to provide temporary power, provision must be made to maintain a discharge voltage at a useable voltage. Unlike classic batteries that cede their entire stored energy on a flat characteristic, the discharge characteristic of the supercapacitor is exponential, the supercapacitor being fully discharged only when a terminal voltage of 0 volts (V) is reached. For example a 30 V nickel-cadmium battery connected to an 18 V bus may discharge all of its energy at or above the 18 V needed to maintain the bus voltage. When a supercapacitor is discharged to the 18 V bus, much of its stored energy will remain in the supercapacitor unless its internal voltage is boosted during the discharge process. In the prior art, this process of boosting requires the use of dedicated hardware such as complex converters. Again, as in the case of charging, use of such dedicated hardware may increase weight and cost of a vehicle.
As can be seen, there is a need to provide an improved supercapacitor-based temporary power source for a power distribution system. In particular, there is a need to provide such a temporary power source which can be operated without dedicated charging hardware. Additionally there is a need to provide such a temporary power source which can be operated without dedicated voltage boosting hardware.